1. Field of the Invention
The present invention relates to a biomedical, implantable electrode for electrically active medical devices. The electrode has an improved surface topography for enhanced electrical performance. Such an electrode is suitable for devices which may be permanently implanted in the human body as stimulation electrodes, for example, as pacemakers, or sensors of medical conditions. This is achieved by the application of ultrafast, high energy pulses to the surface of a solid, monolithic electrode material for the purpose of increasing the surface area and thereby decreasing its after-potential polarization.
2. Description of the Related Art
There is great commercial interest in producing active implantable devices which are typically electrodes used for the stimulation of tissue or the sensing of electrical biorhythms. The electrical performance of implantable electrodes can be enhanced by increasing the external surface area which is in contact with tissues inside the body. It is known that increasing the surface area of an implantable electrode increases the double layer capacitance of the electrode and reduces the after-potential polarization, thereby increasing device battery life, or allowing for lower capture thresholds, and improved sensing of certain electrical signals, such as R and P waves. It is known in the art to apply a coating to increase the surface area of the electrode thereby reducing the after-potential polarization. A reduction in after-potential polarization results in an increase in charge transfer efficiency by allowing increased charge transfer at lower voltages. This is of particular interest in neurological stimulation. Double layer capacitance is typically measured by means of electrochemical impedance spectroscopy. In this method an electrode is submerged in a electrolytic bath and a small cyclic wave is imposed on the electrode. The current and voltage response of the electrode/electrolyte system is measured to determine the double layer capacitance. The capacitance is the predominant factor in the impedance at low frequencies (<10 Hz) and thus the capacitance is typically measured at frequencies of 0.001 Hz-1 Hz.
The current state of the art for increasing the surface area of an implantable electrode is to apply a suitable coating to the surface of electrode substrates. A principal concern in any coating application is the joining of the substrate and coating material and the adhesion between them. In this regard, U.S. Pat. No. 5,571,158 shows a stimulation electrode having a porous surface coating whose active surface area is essentially larger than the surface area defined by the geometrical basic shape of the electrode. U.S. Pat. No. 6,799,076 discloses an electrode having a substrate with a first layer covering at least a portion of the substrate, and a second layer covering at least a portion of the first layer. The first layer consists of a carbide, nitride or carbonitride of titanium, vanadium, zirconium, niobium, molybdenum, hafnium, tantalum or tungsten. The second layer includes iridium. U.S. Pat. No. 5,318,572 teaches a high efficiency tissue stimulating and signal sensing electrode. A lead has a porous electrode of platinum-iridium with recessed areas or grooves formed into the surface. The grooves allow for acute electrode stabilization as a result of clot formation and endocardial tissue capture. At least one layer of a porous coating of 20-200 micron diameter spherical particles are deposited on the surface of the base electrode to obtain a porous macrostructure for promoting chronic tissue ingrowth. A microstructure surface coating is applied to increase the active surface area and enhance electrical efficiency by lowering electrochemical polarization and increasing electrical capacitance.
A particular concern for these techniques is that a section of coating might become dislodged in use and become an irritant. Current techniques for testing the adhesion of a coating to a substrate results in the destruction of the test piece which is costly and requires statistical evidence to validate the test method and sampling. A better alternative to a coating would be the modification of the electrode substrate material itself, thereby eliminating the issue of poor adhesion and the potential of coating particles becoming dislodged during use. Prior attempts to produce a suitable modified surface which does not include a coating have failed due to mechanical limitations. An example is found in U.S. patent publication 2011/0160821 where the surface is laser etched, thus producing ridges with features 25,000 nm to 250,000 nm. For a suitable electrode, the surface features need to be sub-millimeter, for example, from about 1 nm to about 1000 nm. Others have taught laser ablation of electrode surfaces, however, such techniques cannot achieve the nanometer scale feature size of this invention.
The present invention solves these issues by the application of ultra-fast energy pulses supplied to the surface. It has now been found that energy pulses delivered by means of an ultrafast laser produces surface structures on the order of 50 nm to 500 nm which is ideal for tissue stimulation. This process is produced not by laser etching and removal of material but by a restructuring of the surface. In the laser etching process of U.S. patent publication 2011/0160821 the surface is modified through the impingement of the laser, and the smallest feature that can be made equates to the size of the focused laser beam, which is limited by the wavelength of the laser, typically 200-1600 nm.
It has now been found that an important factor in obtaining the desired surface topography for enhanced electrical performance is in the form of a three tiered surface structure. The three structural tiers are described in terms of nano, micro and macro structures. The nano-structures are described as nanoglobules which are manifested as rounded tubes or spherical globules which are almost powdery in appearance but well adhered to the surface. The sphericity of the nanoglobules decreases with an increasing number of laser irradiation pulses per spot. These nanoglobules are superimposed on a hillock-like microstructure in a periodic pattern determined by the wavelength of the laser where the lower the wavelength the smaller the period of the pattern. This microstructure pattern is superimposed on somewhat larger macro structure which resemble ranges of mesas. As discussed more fully below, the macro protrusions have a width in the range of from about 0.15 μm to about 50 μm; the micro protrusions have a width ranging from about 0.15 μm to about 5 μm; and the nano protrusions have a width ranging from about 0.01 μm to about 1 μm. In an embodiment of the invention, the surface may also have voids which extend down into the substrate surface in addition to these outwardly extending protrusions or uplifts.